The present invention concerns the improved process for preparing N-alkyl-3,4-dialkyl-2,6-dinitroanilines. The present invention more particularly concerns a commercially viable process to prepare the compound pendimethalin, (known by the chemical name N-(1-ethylpropyl)-2,6-dinitro-3,4-xylidine) its derivatives and its intermediates.
Pendimethalin and its derivatives are reported by U.S. Pat. No. 4,066,441 as excellent pre-emergent herbicides.
Several procedures are reported in the literature to prepare these compounds. The last step in preparing the compound pendimethalin is reported by U.S. Pat. No. 3,920,742 to involve the nucleophilic substitution of 3,4-dimethyl-2,6-dinitro-halobenzene (especially the chloro derivatives) with the appropriate amine, thus: ##STR2## where the chloro-intermediate was prepared in one of several problematic multi-step processes beginning with o-xylene, as follows:
o-Xylene is first chlorinated to form 4-chloro-o-xylene, which in turn is nitrated with fuming nitric acid to give the 3,4-dimethyl-2,6-dinitro-chlorobenzene in a yield of 27%. First of all, the low yield is not commercially viable. Secondly, the chlorination step affords a mixture of chloro isomers, requiring the distillation of the product mixture to separate the 4-chloro- isomer from its 3-chloro isomer with only great difficulty due to their very close boiling points. Finally, the low yield of the nitration step stems from the need to purify the 3,4-dimethyl-2,6-dinitro-chlorobenzene from dangerous higher nitrated by products. This alone makes this process difficult for commercialization.
A second process, illustrated by U.S. Pat. No. 4,123,250, involves nitrating 3,4-dimethylaniline to form 3,4-dimethyl-2,6-dinitroaniline which is then reacted via a Sandmeyer reaction to form 3,4-dimethyl-2,6-dinitro-chlorobenzene. This process suffers from low yields and requires the laborious separation from various by-products.
A third process, illustrated by U.S. Pat. No. 4,199,669 and U.S. Pat. No. 4,261,926, is based on the reductive alkylation of 3,4-dimethylaniline with a ketone or directly from 4-nitro-o-xylene, thus: ##STR3## and the product is subsequently nitrated with concentrated nitric acid to give the 2,6-dinitro derivative. This process suffers from the fact that the preparation of the 4-nitro-o-xylene always is accompanied by the 3-nitro derivative, and their separation is very difficult.
Several improvements have been reported in attempts to overcome the above described problems. Thus, U.S. Pat. No. 3,929,916 and U.S. Pat. No. 3,929,917 disclosed the use of mercury salts or nitrogen tetroxide to avoid the formation of unwanted by-products. However, these catalysts are poisonous and dangerous materials requiring special handling.
U.S. Pat. No. 4,119,669 and U.S. Pat. No. 4,201,926 report the use of noble metal catalysts. But such catalysts are very expensive with losses during the process.
According to U.S. Pat. No. 4,136,117; and U.S. Pat. No. 4,621,157 the nitration of N-alkyl-xylidine leads to three products:
1. N-Alkyl-3,4-dimethyl-2,6-dinitroaniline:
2. N-Nitroso-N-alkyl-3,4-dimethyl-2,6-dinitroaniline; and
3. N-Nitro-N-alkyl-3,4-dimethyl-2,6-dinitroaniline.
The formation of toxic nitroso compounds makes it imperative to lower the concentration of these compounds to very low levels. As a result, a denitrosation process (as described in U.S. Pat. No. 4,134,917 and U.S. Pat. No. 4,136,117) and denitration process (as described in U.S. Pat. No. 4,391,992) are required in order to obtain commercially useful product. However, it is preferable to use a process to prepare pendimethalin and its analogues which avoids the formation of such toxic nitroso compounds.